Compositions: ceramic – Ceramic compositions – Refractory
Patent
1993-02-23
1994-08-02
Group, Karl
Compositions: ceramic
Ceramic compositions
Refractory
501 94, 501104, 501120, 501153, 264 66, C04B 3548, C04B 3549
Patent
active
053345630
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention concerns refractories. More particularly, it concerns refractory materials in which polycrystalline ceramic particles have been dispersed within a matrix of a refractory material to produce a composite refractory material that exhibits good thermal shock resistance, reasonably high mechanical strength and good resistance to corrosion.
BACKGROUND
In the specification of Australian patent No 591,802, which is essentially the same as WIPO Publication No WO 88/01258, it is pointed out that it is well known that refractory materials cannot be, simultaneously, mechanically strong, dense (which implies good corrosion resistance) and thermal shock resistant. For example, if samples of a dense alumina refractory are heated in a furnace to progressively higher temperatures and then quenched in water to room temperature, and the mechanical strength of the quenched samples is measured, it will be found that at a critical temperature (which varies according to the size of the sample and the conditions under which the experiment is conducted), there is a sudden and significant reduction in the strength retained by the sample after the quenching. In one reported instance, experiments with samples of a dense alumina showed that: following their heating in a furnace to temperatures below 150.degree. C., the modulus of rupture of the quenched samples remained high (about 230 MPa in a test run conducted by the present inventor); thus no deterioration of the mechanical strength of the sample had resulted from the quenching from 150.degree. C. to ambient temperature; furnace temperature of about 150.degree. C., the modulus of rupture of the quenched samples dropped dramatically (to about 60 MPa in the test run), showing that the thermal stresses generated during the quench were sufficient to activate pre-existing surface flaws (cracks) in the alumina refractory, and these flaws had propagated catastrophically through the material, causing the sudden loss of strength of the quenched sample; and excess of about 240.degree. C., the strength of the quenched samples fell approximately exponentially as the temperature drop of the thermal quench increased, the modulus of rupture of the samples reaching a value of about 19 MPa in the test run when the furnace temperature was 400.degree. C.
To reduce the dramatic change of mechanical strength due to unstable crack propagation in the quenching process, the traditional approach has been to introduce porosity into the refractory material. This reduces the low temperature strength of the refractory but the effect of quenching from higher temperatures is less dramatic. For example, samples of the same alumina material that has been described above which contain 5 percent porosity had an inherent low temperature strength of about half that of the dense material, and the low temperature modulus of rupture was approximately 103 MPa. When the samples were heated to the critical temperature of 150.degree. C., quenching to room temperature reduced the strength of the material with 5 percent porosity, but the modulus of rupture of the quenched material was about 87 MPa. Quenching samples of this material to room temperature from a temperature of 400.degree. C. produced samples having a modulus of rupture of about 70 MPa.
A refractory brick which is high in alumina content and has a porosity in the range from 15 percent to 25 percent completely solves the thermal shock problem. The modulus of rupture of quenched samples of such a material varies substantially linearly from about 19 MPa when the material is quenched from low temperatures to about 17 MPa when it is quenched from a temperature of about 400.degree. C. The small loss in mechanical strength when the material is quenched from higher temperatures is due to stable crack propagation. It is clear that with the increase in porosity, the mechanical strength at low temperatures has been sacrificed, being only about one tenth of the strength of a dense commercial alumina ceramic material. However, an even more s
REFERENCES:
patent: 5130277 (1992-07-01), Ueda et al.
Commonwealth Scientific and Industrial Research Organization
Gallo Chris
Group Karl
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